Signal receiver having wide band amplification capability

ABSTRACT

An Improved Signal Receiver Having Wide Band Amplification Capability is disclosed. Also disclosed is a receiver that is able to receive and reliably amplify infrared and/or other wireless signals having frequency bandwidths in excess of 40 MHz. The receiver of the present invention reduces the signal-to-noise ratio of the received signal to ⅕ th  of the prior systems. The preferred receiver eliminates both the shunting resistor and the feedback resistor on the input end by amplifing the signal in current form. Furthermore, the receiver includes transconductance amplification means for amplifying the current signal without the need for Cascode stages. Finally, the receiver includes staged amplification to amplify the current signal in stages prior to converting the signal into a voltage output.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 09/212,203,filed Dec. 15, 1998, now U.S. Pat. No. 6,915,083, the subject matter ofwhich is incorporated herein in its entirety by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to wireless signal transmission systemsand, more specifically, to an Improved Signal Receiver Having Wide BandAmplification Capability.

2. Description of Related Art

In a conventional infrared transceiver system 10 depicted by the diagramof FIG. 1, infrared signals 14 are received by an infrared diode 12.These incident infrared signals 14 generate a current within theinfrared diode 12, which is conventionally converted to a voltage signalby shunting the system with resistor R_(S) as shown. This relativelylow-voltage signal is then passed through a voltage amplifier 16. Thesignal then passes through various stages of staged amplification 18before being carried on out of the system as the output signal V_(IRRX).What should be appreciated is at node V_(OUT) the signal is essentiallythe incident IR signal 14, plus any noise created by the IR diode 12 orthe resistor R_(S). It should be apparent that the better thesignal-to-noise ratio at V_(OUT), the better and cleaner theamplification through the voltage amplifier 16 and the subsequent stagedamplification 18.

Now turning to FIG. 2, we can discuss the operation of the conventionalsystem in more depth. FIG. 2 is a schematic of a single-ended version ofa conventional infrared transceiver system of FIG. 1. As can be seen inFIG. 2, the IR diode 12 is simulated by current source I1 andcapacitance C1. R_(S) of FIG. 1 is here R7, shunted with the currentsource. Essentially, what we have in this diagram is a current mirror 20and a voltage amplifier 22. What should be appreciated from this circuitis that in normal operation the typical input level for fast infrared(FIR) frequency bandwidth will result in approximately 0.5 micro amps ofcurrent at current source I1, which results in 106 micro volts across a“real” 212 ohm resistor R7. Under such conditions, the resistor R7 willhave a thermal noise of 17.8 micro volts (at 40 MHz frequencybandwidth), which results in a noise ratio of 15.5 decibels without evenhaving entered the amplification stages. If we now look at the operationof the amplifier 22, we can see that typically, it is a high impedancevoltage amplifier. The problem with this type of voltage amplifier isthat R7, which is required for the specified system bandwidth, alsoprovides additional noise that is added to the incident infrared signal14 (at V_(OUT)) before the signal is amplified—this further decreasesthe signal-to-noise ratio. It should also be understood that since the“Miller Effect” will apply to the input stage, the value of theintrinsic gate-to-drain capacitance of such a stage is multiplied by thevoltage gain. For example, a voltage gain of 10 will result in a “MillerEffect” drain-to-gate capacity of 11 times. In order to achieve thedesired bandwidth, a Cascode stage becomes a necessity. The addition ofthis Cascode stage results in a corresponding addition of anothertransistor-based noise contribution discussed above (i.e. a total of twoequal noise-contributing stages). Consequently, this phenomena furtherdegrades the signal to noise ratio and harms the amplifier performance.Another type of amplifier has been conventionally used, in which R7 isreplaced by a feedback resistor. This amplifier has not been discussedherein, since its design is limited to a lower bandwidth, in particular,because of its poor noise performance.

Now turning to FIG. 3, we can see a preferred model for the prior artcircuit of FIG. 2. FIG. 3 is a simulation of the circuit of FIG. 2provided for the purposes of modeling the performance of the circuit;the pertinent results of this modeling are shown in FIGS. 4 and 5. FIG.4 is a plot of noise vs. frequency bandwidth for the conventionalcircuit of FIGS. 1 through 3. As can be seen, at a frequency ofapproximately 40 MHz (which is in the FIR bandwidth), the spot noise isapproximately 1.6×10⁻²¹√{square root over (Hz )}. This number willbecome more significant once we discuss the improvements of the presentinvention.

Now turning to FIG. 5 we can see the effect of these noises andcapacitance's created in the prior art voltage feedback typeamplification circuit. FIG. 5 is a response plot of output voltage(V_(IRRX)) for the prior system of FIG. 2. As can be seen, the peaks andvalleys are extremely erratic and choppy, which creates an unstablesignal and ultimately inferior data processing. What is needed is animproved amplifier system to reliably handle in excess of 40 MHzfrequency bandwidth.

SUMMARY OF THE INVENTION

In light of the aforementioned problems associated with the priorsystems and devices, it is an object of the present invention to providean Improved Signal Receiver Having Wide Band Amplification Capability.The preferred receiver should be able to receive and reliably amplifyinfrared and/or other wireless signals having frequency bandwidths inexcess of 40 MHz. It is an object of the present invention to reduce thesignal-to-noise ratio of the received signal to ⅕^(th) of the priorsystems. In its preferred form, the receiver will eliminate bothshunting and feedback resistors on the input end by amplifying thesignal in current form. Furthermore, the receiver will includetransconductance amplification means for amplifying the current signalwithout the need for Cascode stages. It is a further object that thereceiver include staged amplification to amplify the current signal instages prior to converting the signal into a voltage output.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention, which are believed tobe novel, are set forth with particularity in the appended claims. Thepresent invention, both as to its organization and manner of operation,together with further objects and advantages, may best be understood byreference to the following description, taken in connection with theaccompanying drawings, of which:

FIG. 1 is a finctional diagram of a conventional infrared transceiversystem;

FIG. 2 is a schematic of the conventional infrared transceiver system ofFIG. 1;

FIG. 3 is a simulation of the circuit of FIG. 2 provided for thepurposes of modeling the performance of the circuit;

FIG. 4 is a plot of frequency bandwidth of noise to frequency bandwidthfor the conventional circuit of FIGS. 1 through 3;

FIG. 5 a plot of output voltage keeping the effect of the high systemnoise characteristics;

FIG. 6 is a finctional diagram of an improved infrared transceiversystem of the present invention using current amplification;

FIG. 7 is a preferred circuit design of the circuit of FIG. 6;

FIG. 8 is a circuit model of the circuit of FIGS. 6 and 7;

FIG. 9 is a plot of noise versus bandwidth of the circuit of FIGS. 6, 7,and 8; and

FIG. 10 is a plot of output voltage of the circuit of FIGS. 6,7,8, and9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe art to make and use the invention and sets forth the best modescontemplated by the inventors of carrying out their invention. Variousmodifications, however, will remain readily apparent to those skilled inthe art, since the generic principles of the present invention have beendefined herein specifically to provide an Improved Signal ReceiverHaving Wide Band Amplification Capability.

The present invention can best be understood by initial consideration ofFIGS. 6 and 7. FIG. 6 is a functional diagram of an improved infraredtransceiver system 24 of the present invention, employing currentamplification. In this system 24, the IR signals 14 incident upon the hRdiodes remain in the form of a current (I_(OUT)). The Current(I_(OUT−)I_(F)) develops voltage across RINEFF (Effective InputResistance=Rr_(IN)/(1=βA_(OL)). This voltage is multiplied by theTransconductance of the current amplifier 26, producing a currentthrough R_(L), giving a voltage input to the buffer 32. Thispre-amplifier output voltage is converted to a feedback current (I_(F))by device X3. I_(F) is then combined with I_(OUT) which results in areduction in the size of R_(IN) (noiselessly), which ultimately improvesthe bandwidth of the system.

Now turning to FIG. 7, we can see the preferred circuit design for theimproved transceiver system 24 of FIG. 6. As can be seen, in this case,current generator 13 and capacitor C4 simulate the IR diode 12. Incontrast to the prior voltage-type amplifier depicted in FIGS. 2 and 3,the amplifier 30 of this FIG. 7 is a transimpedence-type amplifier.Transistor X3 is connected as a linear amplifier 30 in a feedback stagebetween the circuit's output node 8 and input node 3 to which the outputof the IR diode 12 is also connected. With the transimpedence amplifier,since there is typically no resistive feedback loop (i.e. there is nofeedback resistor), the intrinsic system noise is substantially reduced.Furthermore, the significant benefit of using this topology for thetransimpedence amplifier is that it does not result in a Miller effect,and therefore there is only a noise contribution from a single inputstage (since the full Cascode stage is rendered unnecessary by theabsence of a Miller effect). The result is an amplifier that is capableof extremely high signal-to-noise ratios, in addition to very goodbandwidth, since R_(INEFF) is equal to R_(IN)/(1βA_(OL)).

In order to potentially achieve further performance improvements, thetransistors X3, X6, X7 and/or X4 might include dynamically-adjustablebias voltage control in order to operate these transistors in the “weakinversion” range for certain portions of their operational curves. Sinceweak inversion operations are well known in the art, the particulars ofthis operational mode are not discussed herein. For the purposes of thisdiscussion, a 0.7 μ CMOS process is employed; it should be understoodthat additional system capacitance reductions (and therefore performanceimprovements) might be achievable through the use of smaller geometry.

FIG. 8 is a circuit model of the circuit of FIGS. 6 and 7 constructed inorder to provide simulation data on the circuit, as reported below inFIGS. 9 and 10. FIG. 9 is a plot of noise versus frequency bandwidth ofthe circuit of FIGS. 6, 7, and 8. If we look at the 40 MHz line we cansee that the spot noise at this point is 0.54×10⁻²¹√{square root over(Hz)}. This compares to 1.6×10⁻²¹ of the prior circuit, or approximately1/3 the spot noise at equivalent frequency in the new circuit of FIG. 7(as compared to the old circuit of FIG. 2), which equates to a 13 dBimprovement when integrated over the full frequency range. Also, at 3 dBsignal-to-noise ratio, the frequency bandwidth exceeds 64 MHz.

As can be seen from FIG. 10, the improvement in responsiveness of thetransimpedence solution is dramatic. FIG. 10 is a plot of output voltageof the circuit of FIGS. 6, 7, 8, and 9. In contrast to the sawtoothresponse curve of FIG. 5, FIG. 10 shows a smooth output through severalsignal pulses. It should be understood from FIGS. 9 and 10 that thedevice of the present invention will provide extremely high bandwidthswith low noise while at the same time giving very, very smooth response.It should also be understood that while throughout this application theembodiments discussed have been in regard to infrared signal receipt,this method can also be expected to provide the same benefits for otherwireless signal receipt, for example radio frequency, and in particularcellular phones and other devices. Through application of thistechnology it is believed that the noise improvement of 15 to 16decibels will result in an incredible increase in range and coveragethat heretofore has not been achievable.

Theoretical Noise Comparison to the Prior Art

The following analysis is provided in order to further explain thesignificant benefits of the signal receiver of the present invention. Anoise comparison between the prior art amplifier and the amplifier ofthe present invention revolves around the input transistor and the inputresistor, since the system signal-to-noise ratio is essentiallydetermined at this point in the respective circuits. In the prior artcircuit (see FIG. 2), R7 is the input resistor, and X5 is the inputtransistor. As discussed above, X5 is a Cascode connection. In thepreferred circuit of the present invention, there is NO input resistor,as well as NO Cascode connection.

Input Resistor Contribution

In the prior circuit, assume that a Bandwidth of 40 MHz drives R7 to be265Ω (in order to have adequate gain without decreasing thesignal-to-noise ratio to an unacceptable level). The formula for RMSnoise generated in a resistor is:

${{i_{RMS}({resistor})} = \sqrt{\frac{4 \times k \times T}{R}}},$where:

-   -   k=Boltzman's constant=1.38×10⁻²³    -   T=Temperature (deg. Kelvin)=290    -   R=Resistor value=265        such that:

${{i_{RMS}({R7})} = {\sqrt{\frac{4 \times 1.38 \times 10^{- 23} \times 290}{265}} = 49}},{16{nanoAmperes}}$Input Transistor Contribution

The thernal noise of one input MOSFET is calculated by the followingformula:

${{i_{RMS}({MOSFET})} = \sqrt{\left\{ \frac{8 \times k \times T}{3} \right\} \times \sqrt{2 \times \beta \times {Id}}}},$where:

-   -   β=K′×W/L    -   K′ is a transconductance parameter=30.3×10⁻⁶    -   W/L are width and length dimensions of the MOSFET=55/1        (therefore β=7.575×10⁻⁴)    -   Id is the MOSFET drain current=60×10⁻⁶ (for this case)        such that:

${i_{RMS}({MOSFET})} = {{\sqrt{\left\{ \frac{8 \times 1.38 \times 10^{- 23} \times 290}{3} \right\} \times \sqrt{2 \times 7.575 \times 10^{- 4} \times 60 \times 10^{- 6}}}{i_{RMS}({MOSFET})}} = {11.34{nanoAmperes}}}$Comparison between the Circuits

Assume that the input current source may drop as low as 250 nanoAmperes(fairly common for infrared communications).

The prior circuit's input components' noise:i RMS(input)=i RMS(R7)+i RMS(MOSFET),but since X5 is Cascode-connected, there are essentially two noisecontributions, making the combined contribution equal to the square rootof their squared contributions, therefore:

$\begin{matrix}{{i_{RMS}({input})} = \sqrt{{i_{RMS}({R7})}^{2} + {2 \times \left\{ {i_{RMS}({MOSFET})} \right\}^{2}}}} \\{{i_{RMS}({input})} = {\sqrt{46.16^{2} + {2 \times \left\{ 11.3 \right\}^{2}}} = {51.6{nanoAmpere}\mspace{14mu} s}}}\end{matrix}$

The preferred circuit of the present invention's input components' noise

Since there is no input resistor, the formula for the comparable noisecurrent is simply:i RMS(input)=i RMS(MOSFET)i RMS(input)=11.34 nanoAmperesSignal-to-Noise Ratio Comparison:S:N(prior circuit)=250:51.6=4.85:1S:N(present invention)=250:11.34=22.0:1This represents over 5 (five) times the signal-to-noise ratio of theprior circuit, which, when coupled with the superior frequencyperformance described previously, clearly demonstrates thepreviously-unknown benefits of the present circuit and method over theprior devices and methods.

Those skilled in the art will appreciate that various adaptations andmodifications of the just-described preferred embodiment can beconfigured without departing from the scope and spirit of the invention.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention may be practiced other than as specificallydescribed herein.

1. A device comprising: a receiver of a wireless signal, wherein thereceiver outputs a current signal onto a node; a first transistor with adrain, wherein the node is coupled to the drain of the first transistor,wherein the first transistor generates a feedback current, wherein thefeedback current is dependent on the magnitude of the current signal,and wherein the first transistor is not cascode-connected; a secondtransistor with a source and a drain, wherein the node is coupled to thesource of the second transistor; and a current mirror that is coupled tothe drain of the second transistor, wherein the first transistoramplifies the current signal, wherein the device converts the amplifiedcurrent signal to a voltage signal, and wherein the device does notinclude a feedback resistor.
 2. The device of claim 1, wherein thereceiver outputs the current signal in response to receiving a radiofrequency signal.
 3. The device of claim 1, wherein the device is partof a cellular phone.
 4. The device of claim 1, wherein the receiveroutputs the current signal in response to receiving an infraredradiation signal incident on a photodiode, wherein the infraredradiation signal has a level and the current signal has a magnitude, andwherein the magnitude of the current signal is dependent on the level ofthe infrared radiation signal.
 5. The device of claim 1, wherein thefirst transistor exhibits a bias voltage, and wherein the bias voltageis dynamically adjustable such that the first transistor operates in aweak inversion range.
 6. The device of claim 1, wherein the firsttransistor functions as a transimpedance amplifier.
 7. The device ofclaim 1, wherein the current signal is amplified in stages prior to thedevice converting the amplified current signal to the voltage signal. 8.The device of claim 1, wherein the node exhibits an impedance, andwherein the first transistor reduces the impedance.
 9. The device ofclaim 1, wherein the current signal is amplified over a frequencybandwidth in excess of 40 MHz.
 10. A method comprising: (a) receiving awireless signal; (b) generating a current signal in response toreceiving the wireless signal, wherein the wireless signal has a leveland the current signal has a magnitude, and wherein the magnitude of thecurrent signal is dependent on the level of the wireless signal; (c)amplifying the current signal to generate an amplified current signal,wherein the current signal is amplified in stages, wherein the currentsignal is amplified without using a feedback resistance, wherein thecurrent signal is amplified using a transistor, wherein the transistorexhibits a bias voltage, and wherein the bias voltage is dynamicallyadjustable such that the transistor operates in a weak inversion range;and (d) converting the amplified current signal to a voltage signal. 11.The device of claim 10, wherein the wireless signal is a radio frequencysignal.
 12. The device of claim 10, wherein the wireless signal is aninfrared radiation signal, and wherein the wireless signal is receivedonto a photodiode.
 13. A device comprising: a receiver that outputs acurrent signal onto a node in response to receiving a wireless signal,wherein the wireless signal has a level and the current signal has amagnitude, wherein the magnitude of the current signal is dependent onthe level of the wireless signal, and wherein the node exhibits animpedance; a transistor with a source and a drain, wherein the node iscoupled to the source of the transistor; a current mirror that iscoupled to the drain of the transistor; and means for reducing theimpedance at the node and for amplifying the current signal, wherein thedevice converts the amplified current signal to a voltage signal. 14.The device of claim 13, wherein the means generates a feedback current,wherein the feedback current is dependent on the magnitude of thecurrent signal, and wherein the transistor is not cascode-connected. 15.The device of claim 13, wherein the means amplifies the current signalover a bandwidth, further comprising: means for increasing the bandwidthover which the current signal is amplified.
 16. The device of claim 13,wherein the means does not employ a feedback resistance.
 17. The deviceof claim 13, wherein the means does not employ a cascode-connectedtransistor.
 18. The device of claim 13, wherein the means comprises atransimpedance amplifier.
 19. The device of claim 13, wherein the meansamplifies the current signal in stages prior to converting the amplifiedcurrent signal into the voltage signal.
 20. The device of claim 13,wherein the means employs neither a shunt resistor nor a feedbackresistor.